IP Multimedia Core Network Subsystem (IMS) is an IP-based network architecture proposed by the 3rd Generation Partnership Project (3GPP), which constructs an open and flexible service environment to support multimedia application and provide various multimedia services for users.
In an IMS service system, a control layer is separated from a service layer and provides the service layer with necessary functions such as triggering, routing and charging but not specific services.
In the control layer, a service triggering function and a control function are achieved by a Call Session Control Function (CSCF), which is divided into the following three types: Serving-CSCF (S-CSCF), Proxy-CSCF (P-CSCF), and Interrogating-CSCF (I-CSCF), wherein the S-CSCF plays the major role, and the I-CSCF is optional.
The service layer consisting of a series of Application Servers (ASs) can provide specific services, wherein the AS may be an independent entity or located in an S-CSCF.
The control layer (S-CSCF) controls the triggering of a service according to the subscription information of a user and calls a service of an AS to realize the function of the service. The AS and S-CSCF can both be called Server Equipment (SE).
The end-to-end device used in a session, which is called a User Equipment (UE), takes charge of the interaction with a user. Some UEs can access a network in many ways: for example, via a Packet Switch (PS) domain of a 3GPP, via a PS domain of a non-3GPP, or even via a Circuit Switch (CS) domain.
If a CS network is provided with an enhanced Mobile Switch Center (eMSC) which provides a Session Initial Protocol (SIP) interface to realize an interaction with an IMS network, then the interaction between the IMS network and the CS network can be realized through the eMSC.
For a UE with multiple access modes, if the UE is executing a certain service, such as communication, under a certain access mode that is solely used by the UE at a certain time, then the UE needs to change its access mode after the UE moves to another place, the UE and a network have a capability of providing a certain means to protect the service that is being executed by the UE from being interrupted, such a capability is called single terminal radio voice call continuity, which is called Single Radio Voice Call Continuity (SRVCC) for short.
FIG. 1 is a schematic diagram illustrating an SRVCC, which describes a signaling path and a media path for establishment of a session between a single terminal UE-1 and an IMS terminal UE-2, and a signaling path and a media path between the UE-1 and the UE-2 after an SRVCC occurs. For the sake of a simplified illustration and description, the S-CSCF and the Service Continuity AS (SC AS) are represented as one entity, which communicate with each other using an SIP based on IMS standards.
Before the occurrence of SRVCC, the UE-1 and the UE-2 establishes a session using the signaling paths described below:
A102: the signaling path between the UE-1 and the P-CSCF, which communicate with each other via an SIP of the IMS, the signaling path is an access leg path for the SC AS;
A104: the signaling path between the P-CSCF and the SC AS/S-CSCF, which communicate with each other via the SIP of the IMS, the signaling path is also an access leg path for the SC AS;
R101: the signaling path between the SC AS/S-CSCF and the UE-2, which communicate with each other via the SIP of the IMS, the signaling path is a remote leg path for the SC AS;
the signaling paths and the media paths between the UE-1 and the UE-2 are both changed after an SRVCC occurs, wherein the changes of the signaling paths are described as below:
A112: the signaling path between the UE-1 and the eMSC, which communicate with each other via a signaling protocol of a CS domain, the signaling path is an access leg path for the SC AS;
A114: the signaling path between the eMSC and the SC AS/S-CSCF, which communicate with each other via the SIP of the IMS, and the signaling path is also an access leg path for the SC AS;
R101: the signaling path between the SC AS/S-CSCF and the UE-2, which communicate with each other via the SIP of the IMS, the signaling path is a remote leg path for the SC AS, and the signaling path is unchanged after the occurrence of SRVCC.
FIG. 2 is a diagram illustrating an architecture of an existing SRVCC, in which related parts or net elements of a network participating in realizing an SRVCC and the interfaces or connection relations therebetween are described as below:
description on related net elements:
UE: a user terminal equipment with a capability of SRVCC;
CS network: a network providing conventional CS services for a user;
PS network: a network providing PS services for a user, the control net element of which is a Mobility Management Entity (MME) or a Serving GPRS Support Node (SGSN);
eMSC: the eMSC processes a handover request sent by the control net element of the PS network, executes an inter-domain transfer for a session, and correlates a CS handover operation with the inter-domain transfer operation;
IMS network: a network providing IMS services for a user;
description on related interfaces:
S202: an air interface between the UE and the CS network (CS air interface for short) for realizing an information interaction between the UE and the CS network, such as a standard Um interface;
S204: an air interface between the UE and the control net element of the PS network (PS air interface for short) for realizing an information interaction between the UE and the control net element of the PS network, such as a standard Uu interface;
S206: an interface between the CS network and the eMSC (also called a CS signaling interface), which is changed according to a specific net element connected, for instance, the interface between the eMSC and a base station subsystem is a standard Iu-CS interface, and the interfaces between the eMSC and other mobile switch centers are standard inter-office signaling interfaces, that is, E interface and Nc interface;
S208: a signaling interface between the control net element of the PS network and the eMSC (also called inter-domain handover signaling interface) for supporting an inter-domain handover, this interface is a standard Sv interface;
S210: a signaling interface between the control net element of the PS network and the Internet, such as a standard SGi interface, which is capable of providing an IP data bearer for the information interaction between the UE and the Internet, the IMS network can be counted as a specific Internet as it is based on the Internet;
S212: a signaling path between the eMSC and the IMS network, which may be a standard I2 interface based on the SIP of the IMS between the eMSC and the IMS network or a path constructed by connecting a standard Nc interface between the eMSC and a media gateway and a standard Mg interface between the media gateway and the IMS network; if the path refers to the latter, then the media gateway will interpret a message at the Nc interface into an SIP message of the IMS or vice versa; the Nc interface may be an Nc-SIP interface based on an SIP or an Nc-ISUP interface based on ISDN User Protocol (ISUP). Although the Nc-SIP interface and the I2 interface are both based on the SIP, the SIP only makes regulation on the format of a message but not the content of the message (the content of the message is determined by application), the use of the I2 interface indicates that the eMSC supports IMS-related applications, and the use of the Nc-SIP interface indicates that the eMSC supports conventional CS-related applications.
FIG. 3 is a flow chart of an existing method for realizing an SRVCC, which describes the process that an IMS session between a UE-1 and a UE-2 is established, thereby establishing an IMS media connection path consisting of a media connection between the UE-1 and a control net element of a PS network and a media connection between the control net element of the PS network and the UE-2, and also describes the process that a media connection is established by the UE-1 using a CS domain under the support of the UE-1 and a network while the continuity of the former session is kept after the UE-1 is subjected to an SRVCC. The process comprises the following steps:
step 301: the UE-1 sends a measurement report to the control net element of the PS network serving the UE-1 via an interface S204 between the UE-1 and the control net element of the PS network so as to report the measured cell signal strength information;
step 302: the control net element of the PS network serving the UE-1 (the original control net element of the PS network) determines that the neighboring CS network is more suitable for serving the UE-1 according to the cell signal strength information contained in the measurement report and then determines to carry out a handover operation;
step 303: the original control net element of the PS network (such as an MMS or SGSN) sends, via the interface S208 between the control net element of the PS network and an eMSC, the eMSC a handover request, such as a ‘handover request’ message, the message contains the number information of the UE-1 and the number information of an SC AS for identifying a radio voice call continuity request which is obtained by the control net element of the PS network via a Home Subscriber Server (HSS);
step 304: the eMSC carries out a standard CS handover flow to prepare a media link resource for a target CS network;
step 305: after completing the CS handover flow, the eMSC sends a handover response message to the control net element of the PS network via the interface S208;
step 306: after receiving the handover response message, the control net element of the PS network sends a handover command message to the UE-1 via the interface S204 to inform the UE-1 of performing handover to a CS domain;
step 307: after receiving the handover command message, the UE-1 changes its access mode to be a CS domain access mode;
so far, a CS media connection path is established between the UE-1 and the eMSC, wherein the path consists of a CS media connection between the UE-1 and the CS network and a CS media connection between the CS network and the eMSC;
the following steps follow the step 303 without sequence relationship with steps 304-307;
step 308: after receiving the handover request message sent by the control net element of the PS network, the eMSC sends a call request to the SC AS;
as sent via the signaling path S212 (also called inter-connection and inter-communication signaling path), the call request may be an ‘INVITE’ message of an SIP or an Initial Address Message (IAM) of an ISUP; and the number information of the UE-1 and the number information of the SC AS are contained in the call request, wherein the number information of the SC AS serves as called information, and the number information of the UE-1 serves as calling information;
step 309: the SC AS finally receives the SIP ‘INVITE’ message of the IMS forwarded by a CSCF, determines the message to be a radio voice call continuity request according to the called information, and then searches for the ongoing call related to the current call according to the calling information;
step 310: the SC AS sends an update request of the IMS, such as a ‘UPDATE’ message or a ‘reINVITE’ message, to the UE-2 via the CSCF on the signaling path of the related ongoing call;
step 311: after receiving the update request, the UE-2 responds an update approval message of the IMS, such as a ‘200 OK’ message;
step 312: after receiving the update approval message forwarded by the CSCF, the SC AS sends an answer call message, such as a ‘200 OK’ message, to the eMSC via the signaling path S212, the message finally received by the eMSC may be the ‘200 OK’ message of the SIP or the ANM (ANswer Message) of the ISUP;
so far, a new media path is established between the eMSC and the UE-2, the eMSC connects the new media path with the CS media path to enable the the UE-1 to continue communication with the UE-2.
It can be seen from above that as the SC AS located in the home network carries out no media path anchoring, it is required to perform an update operation to the remote leg in steps 310-311 in the case where existing method for realizing an SRVCC is used, however, as the transmission delay of IMS signaling for the update operation is relatively long, it still takes a long time to establish a new media path even after a CS media is established, thus causing a long interruption in the communication.